将函数及其参数提升到不同的单一上下文 [英] Lift a function and its argument to a different monadic context

问题描述

我不确定如何科学准确地提出这个问题，所以我只是要向你展示一个例子。

I am not sure how to formulate this question scientifically exact, so I am just going to show you an example.

我在 StateT 变压器中使用状态。底层是 IO 。在 StateT IO 操作中，我需要使用 alloca 。但是，我不能提取 alloca 到 StateT IO ，因为它期望类型为的参数。 （Ptr a - > IO a），而我要求它使用参数（Ptr a - > StateT IO MyState a） 。

I am using state in a StateT transformer. Underlying is IO. Inside the StateT IO operation I need to use alloca. However, I can't lift alloca to StateT IO because it expects an argument of type (Ptr a -> IO a) while I require it to work with an argument of (Ptr a -> StateT IO MyState a).

（然而，这是一个关于monad转换器的通用问题，而不是特定于 IO ， StateT 或 alloca 。）

(However, this is a generic question about monad transformers rather than specific to IO, StateT or alloca.)

解决方案：

I came up with the following, working solution:

-- for reference
-- alloca :: (Storable a) => (Ptr a -> IO b) -> IO b

allocaS :: (Storable a) => (Ptr a -> StateT s IO b) -> StateT s IO b
allocaS f = do
  state <- get
  (res, st) <- liftIO $ alloca $ \ptr -> (runStateT (f ptr) state)
  put st
  return res

然而，我认为我应该不得不重构和重构 StateT 动作，以便将它与 alloca 。此外，我已经多次看到这种模式的一些变化，并不总是像 StateT 那样简单和安全。

However, it seems wrong to me that I should have to de- and reconstruct the StateT action in order to use it with alloca. Also, I have seen this pattern in some variations more than once and it's not always as simple and safe as here with StateT.

有没有更好的方法来做到这一点？

Is there a better way to do this?

推荐答案

这可以使用 MonadBaseControl monad-control 中的code> ，已设计正是为此：

This can be accomplished using MonadBaseControl in monad-control, which has been devised exactly for this purpose:

{-# LANGUAGE FlexibleContexts #-}
import Control.Monad
import Control.Monad.Trans.Control
import qualified Foreign.Ptr as F
import qualified Foreign.Marshal.Alloc as F
import qualified Foreign.Storable as F

alloca :: (MonadBaseControl IO m, F.Storable a) => (F.Ptr a -> m b) -> m b 
alloca f = control $ \runInIO -> F.alloca (runInIO . f)

这个增强版本的 alloca 可以用于任何实现 MonadBaseControl 的基于 IO 的monad堆栈，包括 StateT s IO 。

This enhanced version of alloca can be used with any monad stack based on IO that implements MonadBaseControl, including StateT s IO.

MonadBaseControl 的实例允许其monadic值为编码在基本monad中（这里 IO ），传递给基本monad中的函数（如 F.alloca ）然后重新构建它们。

Instances of MonadBaseControl allow their monadic values to be encoded in the base monad (here IO), passed to a function in the base monad (like F.alloca) and then reconstruct them back.

另请参阅 MonadBaseControl的用途是什么？

套餐提升基地包含许多标准的 IO 函数提升到 MonadBaseControl IO ，但是 alloca 不在（其中）。

Package lifted-base contains many of the standard IO functions lifted to MonadBaseControl IO, but alloca isn't (yet) among them.

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